appliance/mariadb: Update the Percona APT signing key
appliance/mariadb: Update the Percona APT signing key

See this article for details on the key rotation.
https://www.percona.com/blog/2016/10/13/new-signing-key-for-percona-debian-and-ubuntu-packages/

Signed-off-by: Joseph Glanville <aed65cdb66c1129f81e57d35b788d35820e08ea4@jpg.id.au>

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